systems a chance to know if they have locked already.
This fixes a locking problem in BFS where one thread tried to acquire two read
locks (where someone else trying to acquire a write lock would have caused a
dead lock).
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@17108 a95241bf-73f2-0310-859d-f6bbb57e9c96